The present invention relates generally to vehicle traction control and, more particularly, to a system for and a method of controlling longitudinal vehicle traction on slippery surfaces and inclines.
It is well known in the industry that vehicle traction is heavily dependent on road surface conditions. However, the ever-changing road surface conditions are almost impossible to continuously monitor and measure, and even harder to apply to vehicle control systems which adjust vehicle braking and traction. Consequently, vehicle traction control devices typically adjust braking and traction based on arbitrarY fixed targets which, since fixed, are at best a compromise for traction control on the various driving surfaces such as loose snow, packed snow, rain, wet leaves, and dry pavement.
In U.S. Pat. No. 4,484,280 issued to Brugger et al, a sYstem directed to preventing undesired spinning of the driven vehicle wheels is disclosed. A threshold value for acceptable wheel spin is preset and the control system is activated only if one or more of the driven wheels is tending to spin at a rate which exceeds the threshold value. The system has a first control loop which reacts to the state of motion of the driven vehicle wheels and, when one of the wheels is tending to spin, that is, the acceptable threshold value has been exceeded, the wheel brake is activated. The system also has a second control loop which causes the output torque of the drive unit to be reduced in the event of a persisting tendency of the driven wheels on both sides of the vehicle to spin. However, the Brugger et al reference does not apply any external estimates, nor does it reflect environmental changes. Thus, the threshold value is not automatically adjusted for changes in external conditions.
In a co-pending, commonly assigned patent application, Ser. No. 07/535,678, filed June 11, 1990 external changes are estimated and made available to a variety of control devices in a vehicle. The estimating device estimates external conditions, most notably the coefficient of friction between a vehicle tire and a roadway, and the slope or grade of the hill the vehicle is attempting to climb or descend. From values of driven wheel spin of the vehicle, non-driven wheel spin of the vehicle, and wheel torque, the estimating device infers the value of the coefficient of friction and the hill slope. Instead of being imbedded in a brake control device or a traction control device, the estimator is a separate device which is capable of providing the determined values to a variety of vehicle control applications.
It is seen then that there is a need for a device which can utilize estimated external values of the coefficient of friction and hill slope in a feedforward manner to control vehicle traction on slippery or changing surfaces and hills to compensate for external disturbances in an optimal manner.